There are various techniques for removing a material (for example, a salt) dissolved in a solvent (for example, water), and in recent years, as a low cost process for energy saving and resource saving, a membrane separation process has been positively used in the water treatment field. Typical membranes used in the membrane separation process include microfiltration membranes, ultrafiltration membranes, nanofiltration membranes (NF membranes) and reverse osmosis membranes (RO membranes).
The RO membranes and NF membranes are mostly composite semipermeable membranes, and most of them are of a type in which a gel layer and a thin layer (separation functional layer) obtained by crosslinking a polymer are formed on a microporous support and of a type in which a thin layer (separation functional layer) obtained by polycondensing a monomer is formed on a microporous support. As the materials of these thin layers, crosslinked polyamides are popularly used. Above all, a composite semipermeable membrane in which a microporous support is covered with a thin layer made of a crosslinked polyamide obtained by a polycondensation reaction between a polyfunctional amine and a polyfunctional acid halide, as described in patent documents 1 and 2, is widely applied as a reverse osmosis membrane or NF membrane, since it is likely to be high in water permeability and salt removal rate.
Economic factors in the water treatment using RO and NF membranes include desalting performance and also ion selective separation performance. For example, in the case where it is desired to transmit monovalent ions and to block divalent ions, if a membrane with low selective separation performance is used, the ion concentration excessively increases on one side of the membrane, to increase the osmotic pressure on the same side of the membrane. If the osmotic pressure on one side increases, more and more ions are going to permeate through the membrane, in order to achieve the balance between the pressures on both sides of the membrane. Consequently a larger pressure is necessary to forcibly make the desalted water permeate through the membrane. As a result, high energy is required to raise the cost of water treatment.
The present RO and/or NF membranes are insufficient in the selective separation performance for separating divalent ions from monovalent ions and are high in the total salt removal rate. Consequently, a high osmic pressure is formed on both sides of each membrane, and in order to achieve a practical flow velocity, a higher pressure, i.e., higher energy is necessary unsatisfactorily in view of energy saving.
On the other hand, in the field of materials, organic/inorganic hybrid materials are known, which are obtained by combining a hydrophilic organic polymer and a condensation product of a silicon compound by utilizing molecular interaction. (see, for example, Patent Documents 3 and 4 and Non-Patent Document 1). However, such materials have not been previously used in industrial applications.